This invention relates generally to integrated circuits, and more specifically, to sample and hold or track and hold circuits.
Sample and hold or track and hold circuits are synonymous terms and are frequently used in electronic circuits where an input signal is sampled and its value is captured or held. Sample and hold circuits are often implemented using operational amplifiers and either a single input structure or a differential input structure is used. An sampling output capacitor is connected to the output of the operational amplifier for storing the sampled input signal and for determining a frequency pole of the circuit, thereby determining frequency stability of the circuit. Coupled to the sampling output capacitor is a buffer amplifier or high impedance circuit that further processes the sampled signal.
The single input structure sample and hold circuit may be implemented with either current sources or voltage sources in an operational amplifier to perform a sample and hold function. An operational amplifier implemented with voltage sources has a low output impedance and is subject to noise from various sources. Noise may be injected into the output signal from the power supply. Additionally, as circuit frequencies have dramatically increased in electronic circuitry a high frequency noise source has been created. When transistors switch at high frequency, such as 1 GHz and higher, the rapid switching transitions of transistors inject a noise signal into the substrate of the integrated circuit. This high frequency noise signal is distributed to other circuitry via the substrate and results in an error. For sample and hold circuitry, noise coupled via the substrate directly results in an error value of the input signal being sampled. Operational amplifiers implemented with voltage sources have little noise immunity, and the substrate voltage noise from high frequency switching is directly coupled onto a sampling capacitor as an error voltage.
An operational amplifier implemented with current sources has a high output impedance, but is also subject to error voltages being coupled onto a sampling capacitor. Because of the high output impedance of such operational amplifiers, such circuits are more commonly used in electronic designs. However, the output terminal of the operational amplifier is subject to having parasitic capacitance to the substrate. The parasitic capacitance functions as a mechanism for coupling any noise injected to the substrate from high frequency transistor switching and is frequency dependent. The high frequency noise is therefore directly coupled to an output sampling capacitor as an error voltage.
Because of the issues mentioned above associated with single input structures used for sample and hold circuits, others have also used a fully differential operational amplifier structure to implement the sample and hold function. However, a fully differential structure requires significantly more circuitry and complexity than a single input structure. For most applications, a single input signal exists. An initial conversion to a differential input signal must be made. The fully differential structure tends to offset error sources that are present in the circuitry, but not all error is completely cancelled. Additionally, in order to sample a single output signal another conversion from a differential output to a single output must be made. Therefore, there is a strong preference to use single input sample and hold circuit.